1. Technical Field
The present invention relates in general to a configuration of a test system for testing integrated circuits on a wafer. More particularly, the present invention relates to a probe card mechanical support configuration for probe cards with low mechanical flexural strength electrical routing substrates.
2. Related Art
With an increased size of wafers, a corresponding increase in size and complexity of test system probe cards for testing the wafers occurs. With the larger wafers, probe card substrates in the wafer test system are typically larger and designed to support more probes, or spring contacts, to connect to and test more integrated circuits (ICs) on the wafers. The larger probe cards with added probes typically result in more bending loads on the probe cards.
While increased wafer sizes are driving larger probe card configurations with a higher probe density, there is a similar move toward use of low flexural strength materials in the probe cards. The move to low flexural strength materials is a result of the need for improved electrical performance of the probe card being demanded by increases in semiconductor device complexity, and the increase in density of ICs per unit area on a wafer being tested. To meet the electrical performance requirements, materials and manufacturing techniques selected for a space transformer substrate supporting probes results in thinner, lower strength configurations. Greater flexing of the probe card substrate is further caused by the increased probe count (load) created with the increased IC density when additional spring contacts of the probe card contact a wafer being tested.
FIG. 1, for reference, shows a simplified block diagram of a test system using a probe card for testing ICs on a semiconductor wafer. The test system includes a test controller 4 connected by a communication cable 6 to a test head 8. The test system further includes a prober 10 made up of a stage 12 for mounting a wafer 14 being tested, the stage 12 being movable to contact the wafer 14 with probes 16 on a probe card 18. The prober 10 includes the probe card 18 supporting probes 16 which contact ICs formed on the wafer 14.
In the test system, test data is generated by the test controller 4 and transmitted through the communication cable 6, test head 8, probe card 18, probes 16 and ultimately to ICs on the wafer 14. Test results are then provided from ICs on the wafer back through the probe card 18 to the test head 8 for transmission back to the test controller 4. Once testing is complete, the wafer is diced up to separate the ICs.
Test data provided from the test controller 4 is divided into the individual tester channels provided through the cable 6 and separated in the test head 8 so that each channel is carried to a separate one of the probes 16. The channels from the test head 8 are linked by connectors 24 to the probe card 18. The connectors 24 can be zero insertion force (ZIF) flexible cable connectors, pogo pins, or other connector types. The probe card 18 then links each channel to a separate one of the probes 16.
FIG. 2 shows a cross sectional view of components of the probe card 18. The probe card 18 is configured to provide both electrical pathways and mechanical support for the spring probes 16 that will directly contact the wafer. The probe card electrical pathways are provided through a printed circuit board (PCB) 30, an interposer 32, and a space transformer 34. Test data from the test head 8 is provided through flexible cable connectors 24 typically connected around the periphery of the PCB 30. Channel transmission lines 40 distribute signals from the connectors 24 horizontally in the PCB 30 to contact pads on the PCB 30 to match the routing pitch of pads on the space transformer 34. The interposer 32 includes a substrate 42 with spring probe electrical contacts 44 disposed on both sides. The interposer 32 electrically connects individual pads on the PCB 30 to pads forming a land grid array (LGA) on the space transformer 34. The LGA pad connections are typically arranged in a regular multi-row pattern. Transmission lines 46 in a substrate 45 of the space transformer 34 distribute signal lines from the LGA to spring probes 16 configured in an array. The space transformer substrate 45 with embedded circuitry, probes and connection points is referred to as a probe head.
Mechanical properties for components providing support for the electrical pathways are dictated by the electrical requirements, since components on a wafer being tested typically operate at a very high frequency. The mechanical support for such a substrate should provide the following:                1. Control of deflection and stress of the space transformer substrate 45.        2. Control of the lateral position for space transformer substrate 45.        3. Precision leveling for space transformer substrate 45.        4. Control of the mechanical compression for the interposer (32) electrical contacts establishing electrical connection between space transformer substrate (45) and PCB (30).        5. Electrical isolation of all unique and bussed electrical circuit structures.        
Mechanical support for the electrical components is provided by a drive plate 50, bracket (Probe Head Bracket) 52, inner frame (Probe Head Frame) 54, interposer 32, and leaf springs 56. The drive plate 50 is provided on one side of the PCB 30, while the bracket 52 is provided on the other side and attached by screws 59. The leaf springs 56 are attached by screws 58 to the bracket 52. The interposer 32 includes two pairs of alignment pins 41 and 43 located in diagonally opposite corners. Pins 43 on the bottom of the interposer are aligned to precision alignment holes in the frame 54 while those on the top align to precision holes in the PCB 30. The positions of the interposer pins 41 and 43 and the alignment holes in the PCB 30 and frame 54 control the lateral motion and hence the alignment of LGA contact pads on the substrate 45 to those on the PCB 30 via the interposer springs 44. The frame 54 further includes horizontal extensions 60 for supporting the space transformer 34 within its interior walls. The bracket 52 and frame 54 provided around the outside edges of the space transformer 34 maintain lateral position control. The probe springs 44 of the interposer 32 then provide a mechanical force separating the PCB 30 and space transformer 34 while holding the frame 54 against the leaf springs 56.
Mechanical components for leveling include four brass spheres (two are shown as 66 and 68) that contact the space transformer 34 near each corner. The brass support spheres provide a point contact outside the periphery of the LGA of the space transformer 34 to maintain isolation from electrical components. Leveling of the substrate is accomplished by adjustment of these spheres through the use of advancing screws (two screws 62 and 64 are shown), referred to as the leveling pins. The leveling pins 62 and 64 are screwed through supports 65 in the drive plate 50 and on both sides of the PCB 30.
Leveling pins 62 and 64 are adjustable to push on the space transformer 34 for both leveling of the space transformer substrate, and to potentially compensate for a substrate which is not planar, or bowed. For leveling, pushing on substrate 45 provided by the leveling pins 62 and 64 will prevent a slight deviation from level from causing spring probes 16 on one side of the space transformer substrate 45 from contacting the wafer, while excessive force is applied between the spring probes 16 and wafer on the other side. For non-planar, bowed or malformed substrates, pushing by leveling pins 62 and 64 can serve to compensate for the malformation. For space transformer substrates with surfaces that are not parallel or planar relative to each other, the leveling pins 62 and 64 are adjusted so that the surface containing the probes is parallel to the wafer surface. For bowed space transformer substrates, pushing provided by leveling pins 62 and 64 at the edges of the substrate can help straighten the bowed shape to some degree. With larger substrates more likely to bow, it is desirable to provide more support structure to compensate for the bowing, such as that described in U.S. Pat. No. 6,509,751 entitled “Planarizer for a Semiconductor Contactor,” which is incorporated herein by reference.
In the past, wafers have been smaller and the number of spring probes on a space transformer have been limited. Thus, a “prober” needed to reposition the wafer to make multiple contacts to the probe card so that all ICs on the wafer could be tested. Typical space transformer substrates used in the construction of wafer probing cards have been relatively rigid (high flexural strength) and the control of deflection and stress has been made possible using the probe card structure shown in FIG. 2
With space transformers having a larger surface area and more pins to test larger wafers requiring fewer touch downs, the space transformer substrates may crack or flex out of a level planar shape due to either forces applied from the interposer, or bending forces resulting from probing. A typical space transformer substrate 45 is constructed from relatively rigid multi-layered ceramic. Using components such as the interposer 32, leveling pins 62 and 64, and frame 54 configured as shown in FIG. 2 leveling has been provided using these rigid ceramic substrates with limited stress. The frame 54 prevents flexure of the substrate in response to the interposer 32, but does nothing to deal with the bending force due to probing, as the space transformer substrate 45 is pushed away from the frame 54 when probing the wafer. With more flexible substrates, it would be desirable to provide increased support to prevent flexing due to interposer forces, as well as bending forces applied during probing.
In the future, softer more flexible substrates such as thin organic based laminates, or membranes may be used in probe cards that have extremely low or relatively no flexural rigidity. It would be desirable to provide a mechanical support configuration for a probe card substrate so that these very low flexural stiffness/strength substrates can be supported without experiencing excessive deflection or stress.